In a gas turbine engine, a combustor discharges hot combustion gases downstream through a high pressure (HP) stationary or stator turbine nozzle which directs the flow between rotor blades of a high pressure turbine (HPT) for extracting energy therefrom. The HPT may have a second stage of rotor blades disposed downstream from the first stage with an additional, second stage turbine nozzle disposed therebetween for channeling the combustion gases from the first stage rotor blades to the second stage rotor blades.
The HP first stage turbine nozzle and the second stage turbine nozzle each include a plurality of circumferentially spaced apart stator vanes or airfoils joined at their radially outer and inner ends to annular outer bands. The nozzles are typically made in arcuate segments with arcuate outer and inner band segments each having one or more vanes per segment. The segments are conventionally joined together to collectively form a complete 360.degree. turbine nozzle.
The HP stage one nozzle is typically supported in the engine at both its outer and inner bands for accommodating loads thereon including pressure forces from the combustion gases channeled between the vanes. However, the second stage nozzle is supported solely at its outer band since a conventional annular seal member is disposed between the first and second rotor stages preventing stationary support of the inner band as well. Accordingly, the vanes of the second stage nozzle are cantilevered from the outer band support which creates bending moments due to the combustion gases flowing between the vanes which must be suitably reacted or accommodated through the outer band.
Since the HP stage one nozzle is supported at both its inner and outer bands, it may be relatively simply manufactured by brazing the vanes at their outer and inner ends to the respective outer and inner bands. However, braze joints have acceptable shear strength but undesirable bending strength. Since the HP nozzle is supported at its outer and inner bands, bending moments from the combustion gases are insignificant, whereas the bending moments in the second stage nozzle are significant since it is supported solely at its outer band, with the inner band thereof being unsupported.
Accordingly, brazed turbine nozzles are typically not used where they cannot be supported at both their outer and inner bands which, therefore, requires alternate and typically more complex and expensive designs.
For example, the vanes may be integrally cast in groups to their outer and inner bands to form integral nozzle segments which do not require brazing between the vanes and bands. Alternatively, a single vane may be integrally cast to outer and inner band segments to avoid brazing therebetween, with the band segments themselves being brazed together at the circumferential joints therebetween. However, these methods of manufacture are not suitable for conventional, high strength, single crystal vanes which are desirable for use in second stage turbine nozzles of improved gas turbine engines.
Accordingly, a simpler and less expensive brazed turbine nozzle is desired for those stages wherein the nozzle is supported solely by its outer band, and which allows the use of single crystal vanes.